Tryptophan hydroxylase from rat midbrain can be activated by high concentrations of Ca ions via a Ca ions-dependent proteolytic reaction. ATP-Mg ions can also activate brain tryptophan hydroxylase via an apparent Ca ions initiated, cAMP-independent protein kinase. In vivid contrast to these results the hydroxylase from malignant mouse mast cell is altered neither by Ca ions nor ATP-Mg ions. In addition, preliminary experiments indicate that the substrate specificities of the two enzymes are also different. Finally, gel filtration experiments revealed that an inhibitor is present in the mast cell preparation. Taken together, these data suggest that hydroxylase enzymes from rat brain and murine mast cell are distinct molecular entities.